Patient temperature regulation method and apparatus

ABSTRACT

A device and method for providing body cooling. The cooling device applies cooling to blood flowing in a vena cavae that is then distributed throughout the body. The cooling can be assisted by use of thermoregulatory drugs or warming devices to prevent shivering and vasoconstriction.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation U.S. patent application Ser. No. 11/340,377,filed on Jan. 25, 2006, entitled “PATIENT TEMPERATURE REGULATION METHODAND APPARATUS, now U.S. Pat. No. 7,311,725, which is a continuation ofU.S. patent application Ser. No. 10/160,611 filed on May 30, 2002,entitled “PATIENT TEMPERATURE REGULATION METHOD AND APPARATUS”, which isa continuation of U.S. patent application Ser. No. 09/373,112, filed onAug. 11, 1999, entitled “PATIENT TEMPERATURE REGULATION METHOD ANDAPPARATUS”, now U.S. Pat. No. 6,843,800, which is a continuation-in-partof U.S. patent application Ser. No. 09/292,532 filed on Apr. 15, 1999,entitled “ISOLATED SELECTIVE ORGAN COOLING METHOD AND APPARATUS”, whichis a continuation-in-part of U.S. patent application Ser. No. 09/052,545filed on Mar. 31, 1998, entitled “CIRCULATING FLUID HYPOTHERMIA METHODAND APPARATUS”, now U.S. Pat. No. 6,231,595; and Ser. No. 09/103,342filed on Jun. 23, 1998, entitled “SELECTIVE ORGAN COOLING CATHETER ANDMETHOD OF USING THE SAME”, now U.S. Pat. No. 6,096,068; the later ofwhich is a continuation-in-part of U.S. Ser. No. 09/047,012 filed Mar.24, 1998, entitled “SELECTIVE ORGAN HYPOTHERMIA METHOD AND APPARATUS”,now U.S. Pat. No. 5,957,963, which is a continuation-in-part of U.S.patent application Ser. No. 09/012,287 filed on Jan. 23, 1998, entitled“SELECTIVE ORGAN HYPOTHERMIA METHOD AND APPARATUS”, now U.S. Pat. No.6,051,019. Each of the prior applications is incorporated herein in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the lowering and control ofthe temperature of the human body. More particularly, the inventionrelates to a method and intravascular apparatus for cooling the bodywithout the adverse consequences associated with prior methods of totalbody cooling. The invention also relates to a method and intravascularapparatus for cooling the body without causing thermoregulatorysuppression of the cooling.

2. Background Information

Organs in the human body, such as the brain, kidney and heart, aremaintained at a constant temperature of approximately 37° C. Hypothermiacan be clinically defined as a core body temperature of 35° C. or less.Hypothermia is sometimes characterized further according to itsseverity. A body core temperature in the range of 33° C. to 35° C. isdescribed as mild hypothermia. A body temperature of 28° C. to 32° C. isdescribed as moderate hypothermia. A body core temperature in the rangeof 24° C. to 28° C. is described as severe hypothermia.

Hypothermia is uniquely effective in reducing brain injury caused by avariety of neurological insults and may eventually play an importantrole in emergency brain resuscitation. Experimental evidence hasdemonstrated that cerebral cooling improves outcome after globalischemia, focal ischemia, or traumatic brain injury. For this reason,hypothermia may be induced in order to reduce the effect of certainbodily injuries to the brain as well as other organs.

Cerebral hypothermia has traditionally been accomplished through wholebody cooling to create a condition of total body hypothermia in therange of 20° C. to 30° C. The currently-employed techniques and devicesused to cause total body hypothermia lead to various side effects. Inaddition to the undesirable side effects, present methods ofadministering total body hypothermia are cumbersome.

Catheters have been developed which are inserted into the bloodstream ofthe patient in order to induce total body hypothermia. For example, U.S.Pat. No. 3,425,419 to Dato describes a method and apparatus of loweringand raising the temperature of the human body. Dato induces moderatehypothermia in a patient using a rigid metallic catheter. The catheterhas an inner passageway through which a fluid, such as water, can becirculated. The catheter is inserted through the femoral vein and thenthrough the inferior vena cava as far as the right atrium and thesuperior vena cava. The Dato catheter has an elongated cylindrical shapeand is constructed from stainless steel. By way of example, Datosuggests the use of a catheter approximately 70 cm in length andapproximately 6 mm in diameter. Thus, the Dato device cools along thelength of a very elongated device. Use of the Dato device is highlycumbersome due to its size and lack of flexibility.

U.S. Pat. No. 5,837,003 to Ginsburg also discloses a method andapparatus for controlling a patient's body temperature. In thistechnique, a flexible catheter is inserted into the femoral artery orvein or the jugular vein. The catheter may be in the form of a balloonto allow an enhanced surface area for heat transfer. A thermallyconductive metal foil may be used as part of a heat-absorbing surface.This device fails to disclose or teach use of any ability to enhanceheat transfer. In addition, the disclosed device fails to disclosetemperature regulation.

Therefore, a practical method and apparatus that lowers and controls thetemperature of the human body satisfies a long-felt need.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the apparatus of the present invention can include a heattransfer element that can be used to apply cooling to the blood flowingin a large vein feeding the heart. An optional heating element may beused to supply warming to a portion of the remainder of the body toprovide comfort to the patient and to allow a low target hypothermictemperature to be achieved. The heating element may be applied before orafter a target temperature is achieved. The warming operation can beaccomplished by means of local heating of the vein or artery with anexternal heat applicator or by means of substantially whole body warmingwith a heating blanket. The warming operation can be accomplished per seor in combination with thermoregulatory drugs.

The heat transfer element, by way of example only, includes first andsecond elongated, articulated segments, each segment having amixing-inducing exterior surface. A flexible joint can connect the firstand second elongated segments. An inner lumen may be disposed within thefirst and second elongated segments and is capable of transporting apressurized working fluid to a distal end of the first elongatedsegment. In addition, the first and second elongated segments may have amixing-inducing interior surface for inducing mixing within thepressurized working fluid. The mixing-inducing exterior surface may beadapted to induce mixing within a blood flow when placed within anartery or vein. In one embodiment, the flexible joint includes a bellowssection that also allows for axial compression of the heat transferelement as well as for enhanced flexibility. In alternative embodiments,the bellows section may be replaced with flexible tubing such as smallcylindrical polymer connecting tubes.

In one embodiment, the mixing-inducing exterior'surfaces of the heattransfer element include one or more helical grooves and ridges.Adjacent segments of the heat transfer element can be oppositelyspiraled to increase mixing. For instance, the first elongated heattransfer segment may include one or more helical ridges having acounter-clockwise twist, while the second elongated heat transfersegment includes one or more helical ridges having a clockwise twist.Alternatively, of course, the first elongated heat transfer segment mayinclude one or more clockwise helical ridges, and the second elongatedheat transfer segment may include one or more counter-clockwise helicalridges. The first and second elongated, articulated segments may beformed from highly conductive materials such as metals.

The heat transfer device may also have a supply catheter with an innercatheter lumen coupled to the inner lumen within the first and secondelongated heat transfer segments. A working fluid supply configured todispense the pressurized working fluid may be coupled to the innercatheter lumen or alternatively to the supply catheter. The workingfluid supply may be configured to produce the pressurized working fluidat a temperature of about 0° C. and at a pressure below about 5atmospheres of pressure.

In yet another alternative embodiment, the heat transfer device may havethree or more elongated, articulated, heat transfer segments each havinga mixing-inducing exterior surface, with additional flexible jointsconnecting the additional elongated heat transfer segments. In one suchembodiment, by way of example only, the first and third elongated heattransfer segments may include clockwise helical ridges, and the secondelongated heat transfer segment may include one or morecounter-clockwise helical ridges. Alternatively, of course, the firstand third elongated heat transfer segments may include counter-clockwisehelical ridges, and the second elongated heat transfer segment mayinclude one or more clockwise helical ridges.

The mixing-inducing exterior surface of the heat transfer element mayoptionally include a surface coating or treatment to inhibit clotformation. A surface coating may also be used to provide a degree oflubricity to the heat transfer element and its associated catheter.

The present invention is also directed to a method of inducinghypothermia in the body by inserting a flexible, conductive coolingelement into a vein that is in pressure communication with the heart,e.g., the superior or inferior vena cavae or both. The vena cavae may beaccessed via known techniques from the jugular vein or from thesubclavian or femoral veins, for example. The heat transfer element inone or both vena cavae may then cool virtually all the blood beingreturned to the heart. The cooled blood enters the right atrium at whichpoint the same is pumped through the right ventricle and into thepulmonary artery to the lungs where the same is oxygenated. Due to theheat capacity of the lungs, the blood does not appreciably warm duringoxygenation. The cooled blood is returned to the heart and pumped to theentire body via the aorta. Thus, cooled blood may be deliveredindirectly to a chosen organ such as the brain. This indirect cooling isespecially effective as high blood flow organs such as the heart andbrain are preferentially supplied blood by the vasculature. A warmingblanket or other warming device may be applied to portions of the bodyto provide comfort to the patient and to inhibit thermoregulatoryresponses such as vasoconstriction. Thermoregulatory drugs may also beso provided for this reason.

The method further includes circulating a working fluid through theflexible, conductive cooling element in order to lower the temperatureof the blood in the vena cava. The flexible, conductive heat transferelement preferably absorbs more than about 150 or 300 Watts of heat.

The method may also include inducing mixing within the free stream bloodflow within the vena cava. It is noted that a degree of turbulence ormixing is generally present within the vena cava anyway. The step ofcirculating may include inducing mixing in the flow of the working fluidthrough the flexible, conductive heat transfer element. The pressure ofthe working fluid may be maintained below about 5 atmospheres ofpressure.

The present invention also envisions a method for inducing therapeutichypothermia in the body of a patient which includes introducing acatheter, with a cooling element, into a vena cava supplying the heart,the catheter having a diameter of about 18 mm or less, inducing mixingin blood flowing over the cooling element, and lowering the temperatureof the cooling element to remove heat from the blood to cool the blood.In one embodiment, the cooling step removes at least about 150 Watts ofheat from the blood. In another embodiment, the cooling step removes atleast about 300 Watts of heat from the blood.

The mixing induced may result in a Nusselt number enhancement of theflow of between about 5 and 80.

In another aspect of the method, the invention is directed to a methodof lowering the temperature of the body while prohibiting interventionof the body's thermoregulatory responses. Steps of the method mayinclude delivering a drug to lower the thermoregulatory setpoint of thebody such that thermoregulatory responses, including shivering andvasoconstriction, are not triggered above a certain temperature, whereinthe certain temperature is lower than normal body temperature. Thetemperature of the blood in a major vein such as the vena cavae is thenlowered to induce hypothermia in the body. The thermoregulatory drugsprovide patient comfort. If even lower body temperatures are desired orrequired, heating blankets may be provided to further ensure patientcomfort. Generally, for one degree of body core cooling, the heatingblanket should be 5° C. above the skin temperature to provide patientcomfort. However, the temperature of the blanket should generally notexceed 42° C.

Advantages of the invention are numerous. Patients can be provided withthe beneficial aspects of hypothermia without suffering the deleteriousconsequences of the prior art. The procedure can be administered safelyand easily. Numerous cardiac and neural settings can benefit by thehypothermic therapy. For example, ischemia and re-stenosis can beminimized. Other advantages will be understood from the following.

The novel features of this invention, as well as the invention itself,will be best understood from the attached drawings, taken along with thefollowing description, in which similar reference characters refer tosimilar parts, and in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is an elevation view of one embodiment of a heat transfer elementaccording to the invention;

FIG. 2 is a longitudinal section view of the heat transfer element ofFIG. 1;

FIG. 3 is a transverse section view of the heat transfer element of FIG.1;

FIG. 4 is a perspective view of the heat transfer element of FIG. 1 inuse within a blood vessel;

FIG. 5 is a cut-away perspective view of an alternative embodiment of aheat transfer element according to the invention;

FIG. 6 is a transverse section view of the heat transfer element of FIG.5;

FIG. 7 is a schematic representation of the heat transfer element beingused in one embodiment to provide hypothermia to a patient by causingtotal body cooling and then rewarming the body;

FIG. 8 is a schematic representation of the heat transfer element beingused in another embodiment to provide hypothermia to a patient bycausing total body cooling and then rewarming the body;

FIG. 9 is a schematic representation of the heat transfer element beingused in an embodiment within the superior vena cava;

FIG. 10 is a graph showing preferential cooling of the high flow organsof the body under a hypothermic therapy; and

FIG. 11 is a flowchart showing an exemplary method of the inventionemploying heating blankets and thermoregulatory drugs.

DETAILED DESCRIPTION OF THE INVENTION Overview

A one or two-step process and a one or two-piece device may be employedto intravascularly lower the temperature of a body in order to inducetherapeutic hypothermia. A cooling element may be placed in a high-flowvein such as the vena cavae to absorb heat from the blood flowing intothe heart. This transfer of heat causes a cooling of the blood flowingthrough the heart and thus throughout the vasculature. Such a method anddevice may therapeutically be used to induce an artificial state ofhypothermia.

A heat transfer element that systemically cools blood should be capableof providing the necessary heat transfer rate to produce the desiredcooling effect throughout the vasculature. This may be up to or greaterthan 300 watts, and is at least partially dependent on the mass of thepatient and the rate of blood flow. Surface features may be employed onthe heat transfer element to enhance the heat transfer rate. The surfacefeatures and other components of the heat transfer element are describedin more detail below.

One problem with hypothermia as a therapy is that the patient'sthermoregulatory defenses initiate, attempting to defeat thehypothermia. Methods and devices may be used to lessen thethermoregulatory response. For example, a heating blanket may cover thepatient. In this way, the patient may be made more comfortable.Thermoregulatory drugs may also be employed to lower the trigger pointat which the patient's thermoregulatory system begins to initiatedefenses. Such drugs are described in more detail below. A methodemploying thermoregulatory drugs, heating blankets, and heat transferelements is also disclosed below.

Anatomical Placement

The internal jugular vein is the vein that directly drains the brain.The external jugular joins the internal jugular at the base of the neck.The internal jugular veins join the subclavian veins to form thebrachiocephalic veins that in turn drain into the superior vena cava.The superior vena cava drains into the right atrium of the heart as maybe seen by referring ahead to FIG. 9. The superior vena cava suppliesblood to the heart from the upper part of the body.

A cooling element may be placed into the superior vena cava, inferiorvena cava, or otherwise into a vein which feeds into the superior venacava or otherwise into the heart to cool the body. A physicianpercutaneously places the catheter into the subclavian or internal orexternal jugular veins to access the superior vena cava. The blood,cooled by the heat transfer element, may be processed by the heart andprovided to the body in oxygenated form to be used as a conductivemedium to cool the body. The lungs have a fairly low heat capacity, andthus the lungs do not cause appreciable rewarming of the flowing blood.

The vasculature by its very nature provides preferential blood flow tothe high blood flow organs such as the brain and the heart. Thus, theseorgans are preferentially cooled by such a procedure as is also shownexperimentally in FIG. 10. FIG. 10 is a graph of measured temperatureplotted versus cooling time. This graph show the effect of placing acooling element in the superior vena cavae of a sheep. The core bodytemperature as measured by an esophageal probe is shown by curve 82. Thebrain temperature is shown by curve 86. The brain temperature is seen todecrease more rapidly than the core body temperature throughout theexperiment. The inventors believe this effect to be due to thepreferential supply of blood provided to the brain and heart. Thiseffect may be even more pronounced if thermoregulatory effects, such asvasoconstriction, occur that tend to focus blood supply to the corevascular system and away from the peripheral vascular system.

Heat Transfer

When a heat transfer element is inserted approximately coaxially into anartery or vein, the primary mechanism of heat transfer between thesurface of the heat transfer element and the blood is forced convection.Convection relies upon the movement of fluid to transfer heat. Forcedconvection results when an external force causes motion within thefluid. In the case of arterial or venous flow, the beating heart causesthe motion of the blood around the heat transfer element.

The magnitude of the heat transfer rate is proportional to the surfacearea of the heat transfer element, the temperature differential, and theheat transfer coefficient of the heat transfer element.

The receiving artery or vein into which the heat transfer element isplaced has a limited diameter and length. Thus, the surface area of theheat transfer element must be limited to avoid significant obstructionof the artery or vein and to allow the heat transfer element to easilypass through the vascular system. For placement within the superior venacava via the external jugular, the cross sectional diameter of the heattransfer element may be limited to about 5-6 mm, and its length may belimited to approximately 10-15 cm. For placement within the inferiorvena cava, the cross sectional diameter of the heat transfer element maybe limited to about 6-7 mm, and its length may be limited toapproximately 25-35 cm.

Decreasing the surface temperature of the heat transfer element canincrease the temperature differential. However, the minimum allowablesurface temperature is limited by the characteristics of blood. Bloodfreezes at approximately 0° C. When the blood approaches freezing, iceemboli may form in the blood, which may lodge downstream, causingserious ischemic injury. Furthermore, reducing the temperature of theblood also increases its viscosity, which results in a small decrease inthe value of the convection heat transfer coefficient. In addition,increased viscosity of the blood may result in an increase in thepressure drop within the artery, thus compromising the flow of blood tothe brain. Given the above constraints, it is advantageous to limit theminimum allowable surface temperature of the cooling element toapproximately 5° C. This results in a maximum temperature differentialbetween the blood stream and the cooling element of approximately 32° C.For other physiological reasons, there are limits on the maximumallowable surface temperature of the warming element.

The mechanisms by which the value of the convection heat transfercoefficient may be increased are complex. However, it is well known thatthe convection heat transfer coefficient increases with the level of“mixing” or “turbulent” kinetic energy in the fluid flow. Thus it isadvantageous to have blood flow with a high degree of mixing in contactwith the heat transfer element.

The blood flow has a considerably more stable flux in the superior venacava than in an artery. However, the blood flow in the superior venacava still has a high degree of inherent mixing or turbulence. Reynoldsnumbers in the superior vena cava may range, for example, from 2,000 to5,000. Thus, blood cooling in the superior vena cava may benefit fromenhancing the level of mixing with the heat transfer element but thisbenefit may be substantially less than that caused by the inherentmixing.

Boundary Layers

A thin boundary layer has been shown to form during the cardiac cycle.Boundary layers develop adjacent to the heat transfer element as well asnext to the walls of the artery or vein. Each of these boundary layershas approximately the same thickness as the boundary layer that wouldhave developed at the wall of the artery in the absence of the heattransfer element. The free stream flow region is developed in an annularring around the heat transfer element. The heat transfer element used insuch a vessel should reduce the formation of such viscous boundarylayers.

Heat Transfer Element Characteristics and Description

The intravascular heat transfer element should be flexible in order tobe placed within the vena cavae or other veins or arteries. Theflexibility of the heat transfer element is an important characteristicbecause the same is typically inserted into a vein such as the externaljugular and accesses the superior vena cava by initially passing thougha series of one or more branches. Further, the heat transfer element isideally constructed from a highly thermally conductive material such asmetal in order to facilitate heat transfer. The use of a highlythermally conductive material increases the heat transfer rate for agiven temperature differential between the working fluid within the heattransfer element and the blood. This facilitates the use of a highertemperature coolant, or lower temperature warming fluid, within the heattransfer element, allowing safer working fluids, such as water orsaline, to be used. Highly thermally conductive materials, such asmetals, tend to be rigid. Therefore, the design of the heat transferelement should facilitate flexibility in an inherently inflexiblematerial.

It is estimated that the cooling element should absorb at least about300 Watts of heat when placed in the superior vena cava to lower thetemperature of the body to between about 30° C. and 34° C. Thesetemperatures are thought to be appropriate to obtain the benefits ofhypothermia described above. The power removed determines how quicklythe target temperature can be reached. For example, in a stroke therapyin which it is desired to lower brain temperature, the same may belowered about 4° C. per hour in a 70 kg human upon removal of 300 Watts.

One embodiment of the invention uses a modular design. This designcreates helical blood flow and produces a level of mixing in the bloodflow by periodically forcing abrupt changes in the direction of thehelical blood flow. The abrupt changes in flow direction are achievedthrough the use of a series of two or more heat transfer segments, eachincluded of one or more helical ridges. The use of periodic abruptchanges in the helical direction of the blood flow in order to inducestrong free stream turbulence may be illustrated with reference to acommon clothes washing machine. The rotor of a washing machine spinsinitially in one direction causing laminar flow. When the rotor abruptlyreverses direction, significant turbulent kinetic energy is createdwithin the entire wash basin as the changing currents cause randomturbulent motion within the clothes-water slurry. These surface featuresalso tend to increase the surface area of the heat transfer element,further enhancing heat transfer.

FIG. 1 is an elevation view of one embodiment of a cooling element 14according to the present invention. The heat transfer element 14includes a series of elongated, articulated segments or modules 20, 22,24. Three such segments are shown in this embodiment, but two or moresuch segments could be used without departing from the spirit of theinvention. As seen in FIG. 1, a first elongated heat transfer segment 20is located at the proximal end of the heat transfer element 14. Amixing-inducing exterior surface of the segment 20 includes fourparallel helical ridges 28 with four parallel helical grooves 26therebetween. One, two, three, or more parallel helical ridges 28 couldalso be used without departing from the spirit of the present invention.In this embodiment, the helical ridges 28 and the helical grooves 26 ofthe heat transfer segment 20 have a left hand twist, referred to hereinas a counter-clockwise spiral or helical rotation, as they proceedtoward the distal end of the heat transfer segment 20.

The first heat transfer segment 20 is coupled to a second elongated heattransfer segment 22 by a first bellows section 25, which providesflexibility and compressibility. The second heat transfer segment 22includes one or more helical ridges 32 with one or more helical grooves30 therebetween. The ridges 32 and grooves 30 have a right hand, orclockwise, twist as they proceed toward the distal end of the heattransfer segment 22. The second heat transfer segment 22 is coupled to athird elongated heat transfer segment 24 by a second bellows section 27.The third heat transfer segment 24 includes one or more helical ridges36 with one or more helical grooves 34 therebetween. The helical ridge36 and the helical groove 34 have a left hand, or counter-clockwise,twist as they proceed toward the distal end of the heat transfer segment24. Thus, successive heat transfer segments 20, 22, 24 of the heattransfer element 14 alternate between having clockwise andcounterclockwise helical twists. The actual left or right hand twist ofany particular segment is immaterial, as long as adjacent segments haveopposite helical twist.

In addition, the rounded contours of the ridges 28, 32, 36 allow theheat transfer element 14 to maintain a relatively atraumatic profile,thereby minimizing the possibility of damage to the blood vessel wall. Aheat transfer element according to the present invention may includetwo, three, or more heat transfer segments.

The bellows sections 25, 27 are formed from seamless and nonporousmaterials, such as metal, and therefore are impermeable to gas, whichcan be particularly important, depending on the type of working fluidthat is cycled through the heat transfer element 14. The structure ofthe bellows sections 25, 27 allows them to bend, extend and compress,which increases the flexibility of the heat transfer element 14 so thatit is more readily able to navigate through blood vessels. The bellowssections 25, 27 also provide for axial compression of the heat transferelement 14, which can limit the trauma when the distal end of the heattransfer element 14 abuts a blood vessel wall. The bellows sections 25,27 are also able to tolerate cryogenic temperatures without a loss ofperformance. In alternative embodiments, the bellows may be replaced byflexible polymer tubes, which are bonded between adjacent heat transfersegments.

The exterior surfaces of the heat transfer element 14 can be made frommetal, and may include very high thermal conductivity materials such asnickel, thereby facilitating heat transfer. Alternatively, other metalssuch as stainless steel, titanium, aluminum, silver, copper and thelike, can be used, with or without an appropriate coating or treatmentto enhance biocompatibility or inhibit clot formation. Suitablebiocompatible coatings include, e.g., gold, platinum or polymerparalyene. The heat transfer element 14 may be manufactured by plating athin layer of metal on a mandrel that has the appropriate pattern. Inthis way, the heat transfer element 14 may be manufactured inexpensivelyin large quantities, which is an important feature in a disposablemedical device.

Because the heat transfer element 14 may dwell within the blood vesselfor extended periods of time, such as 24-48 hours or even longer, it maybe desirable to treat the surfaces of the heat transfer element 14 toavoid clot formation. In particular, one may wish to treat the bellowssections 25, 27 because stagnation of the blood flow may occur in theconvolutions, thus allowing clots to form and cling to the surface toform a thrombus. One means by which to prevent thrombus formation is tobind an antithrombogenic agent to the surface of the heat transferelement 14. For example, heparin is known to inhibit clot formation andis also known to be useful as a biocoating. Alternatively, the surfacesof the heat transfer element 14 may be bombarded with ions such asnitrogen. Bombardment with nitrogen can harden and smooth the surfaceand thus prevent adherence of clotting factors. Another coating thatprovides beneficial properties may be a lubricious coating. Lubriciouscoatings, on both the heat transfer element and its associated catheter,allow for easier placement in the, e.g., vena cava.

FIG. 2 is a longitudinal sectional view of the heat transfer element 14of an embodiment of the invention, taken along line 2-2 in FIG. 1. Someinterior contours are omitted for purposes of clarity. An inner tube 42creates an inner lumen 40 and an outer lumen 46 within the heat transferelement 14. Once the heat transfer element 14 is in place in the bloodvessel, a working fluid such as saline or other aqueous solution may becirculated through the heat transfer element 14. Fluid flows up a supplycatheter into the inner lumen 40. At the distal end of the heat transferelement 14, the working fluid exits the inner lumen 40 and enters theouter lumen 46. As the working fluid flows through the outer lumen 46,heat is transferred from the working fluid to the exterior surface 37 ofthe heat transfer element 14. Because the heat transfer element 14 isconstructed from a high conductivity material, the temperature of itsexterior surface 37 may reach very close to the temperature of theworking fluid. The tube 42 may be formed as an insulating divider tothermally separate the inner lumen 40 from the outer lumen 46. Forexample, insulation may be achieved by creating longitudinal airchannels in the wall of the insulating tube 42. Alternatively, theinsulating tube 42 may be constructed of a non-thermally conductivematerial like polytetrafluoroethylene or another polymer.

It is important to note that the same mechanisms that govern the heattransfer rate between the exterior surface 37 of the heat transferelement 14 and the blood also govern the heat transfer rate between theworking fluid and the interior surface 38 of the heat transfer element14. The heat transfer characteristics of the interior surface 38 areparticularly important when using water, saline or other fluid thatremains a liquid as the working fluid. Other coolants such as Freonundergo nucleate boiling and create mixing through a differentmechanism. Saline is a safe working fluid, because it is non-toxic, andleakage of saline does not result in a gas embolism, which could occurwith the use of boiling refrigerants. Since mixing in the working fluidis enhanced by the shape of the interior surface 38 of the heat transferelement 14, the working fluid can be delivered to the cooling element 14at a warmer temperature and still achieve the necessary cooling rate.Similarly, since mixing in the working fluid is enhanced by the shape ofthe interior surface of the heat transfer element, the working fluid canbe delivered to the warming element 14 at a cooler temperature and stillachieve the necessary warming rate.

This has a number of beneficial implications in the need for insulationalong the catheter shaft length. Due to the decreased need forinsulation, the catheter shaft diameter can be made smaller. Theenhanced heat transfer characteristics of the interior surface of theheat transfer element 14 also allow the working fluid to be delivered tothe heat transfer element 14 at lower flow rates and lower pressures.High pressures may make the heat transfer element stiff and cause it topush against the wall of the blood vessel, thereby shielding part of theexterior surface 37 of the heat transfer element 14 from the blood.Because of the increased heat transfer characteristics achieved by thealternating helical ridges 28, 32, 36, the pressure of the working fluidmay be as low as 5 atmospheres, 3 atmospheres, 2 atmospheres or evenless than 1 atmosphere.

FIG. 3 is a transverse sectional view of the heat transfer element 14 ofthe invention, taken at a location denoted by the line 3-3 in FIG. 1.FIG. 3 illustrates a five-lobed embodiment, whereas FIG. 1 illustrates afour-lobed embodiment. As mentioned earlier, any number of lobes mightbe used. In FIG. 3, the construction of the heat transfer element 14 isclearly shown. The inner lumen 40 is defined by the insulating tube 42.The outer lumen 46 is defined by the exterior surface of the insulatingtube 42 and the interior surface 38 of the heat transfer element 14. Inaddition, the helical ridges 32 and helical grooves 30 may be seen inFIG. 3. Although FIG. 3 shows four ridges and four grooves, the numberof ridges and grooves may vary. Thus, heat transfer elements with 1, 2,3, 4, 5, 6, 7, 8 or more ridges are specifically contemplated.

FIG. 4 is a perspective view of a heat transfer element 14 in use withina blood vessel, showing only one helical lobe per segment for purposesof clarity. Beginning from the proximal end of the heat transfer element(not shown in FIG. 4), as the blood moves forward, the first helicalheat transfer segment 20 induces a counter-clockwise rotational inertiato the blood. As the blood reaches the second segment 22, the rotationaldirection of the inertia is reversed, causing mixing within the blood.Further, as the blood reaches the third segment 24; the rotationaldirection of the inertia is again reversed. The sudden changes in flowdirection actively reorient and randomize the velocity vectors, thusensuring mixing throughout the bloodstream. During such mixing, thevelocity vectors of the blood become more random and, in some cases,become perpendicular to the axis of the vessel. Thus, a large portion ofthe volume of warm blood in the vessel is actively brought in contactwith the heat transfer element 14, where it can be cooled by directcontact rather than being cooled largely by conduction through adjacentlaminar layers of blood.

Referring back to FIG. 1, the heat transfer element 14 has been designedto address all of the design criteria discussed above. First, the heattransfer element 14 is flexible and is made of a highly conductivematerial. The flexibility is provided by a segmental distribution ofbellows sections 25, 27 that provide an articulating mechanism. Bellowshave a known convoluted design that provide flexibility. Second, theexterior surface area 37 has been increased through the use of helicalridges 28, 32, 36 and helical grooves 26, 30, 34. The ridges also allowthe heat transfer element 14 to maintain a relatively atraumaticprofile, thereby minimizing the possibility of damage to the vesselwall. Third, the heat transfer element 14 has been designed to promotemixing both internally and externally. The modular or segmental designallows the direction of the grooves to be reversed between segments. Thealternating helical rotations create an alternating flow that results inmixing the blood in a manner analogous to the mixing action created bythe rotor of a washing machine that switches directions back and forth.This action is intended to promote mixing to enhance the heat transferrate. The alternating helical design also causes beneficial mixing, orturbulent kinetic energy, of the working fluid flowing internally.

FIG. 5 is a cut-away perspective view of an alternative embodiment of aheat transfer element 50. An external surface 52 of the heat transferelement 50 is covered with a series of axially staggered protrusions 54.The staggered nature of the outer protrusions 54 is readily seen withreference to FIG. 6 which is a transverse cross-sectional view taken ata location denoted by the line 6-6 in FIG. 5. As the blood flows alongthe external surface 52, it collides with one of the staggeredprotrusions 54 and a turbulent wake flow is created behind theprotrusion. As the blood divides and swirls alongside of the firststaggered protrusion 54, its turbulent wake encounters another staggeredprotrusion 54 within its path preventing the re-lamination of the flowand creating yet more mixing. In this way, the velocity vectors arerandomized and mixing is created not only in the boundary layer but alsothroughout a large portion of the free stream. As is the case with thepreferred embodiment, this geometry also induces a mixing effect on theinternal working fluid flow.

A working fluid is circulated up through an inner lumen 56 defined by aninsulating tube 58 to a distal tip of the heat transfer element 50. Theworking fluid then traverses an outer lumen 60 in order to transfer heatto the exterior surface 52 of the heat transfer element 50. The insidesurface of the heat transfer element 50 is similar to the exteriorsurface 52 in order to induce turbulent flow of the working fluid. Theinner protrusions can be aligned with the outer protrusions 54 as shownin FIG. 6 or they can be offset from the outer protrusions 54 as shownin FIG. 5.

Method of Use

FIG. 7 is a schematic representation of the invention being used to coolthe body of a patient and to warm a portion of the body. The hypothermiaapparatus shown in FIG. 7 includes a first working fluid supply 10,preferably supplying a chilled liquid such as water, alcohol or ahalogenated hydrocarbon, a first supply catheter 12 and the coolingelement 14. The first supply catheter 12 may have a substantiallycoaxial construction. An inner lumen within the first supply catheter 12receives coolant from the first working fluid supply 10. The coolanttravels the length of the first supply catheter 12 to the coolingelement 14 which serves as the cooling tip of the catheter. At thedistal end of the cooling element 14, the coolant exits the insulatedinterior lumen and traverses the length of the cooling element 14 inorder to decrease the temperature of the cooling element 14. The coolantthen traverses an outer lumen of the first supply catheter 12 so that itmay be disposed of or recirculated. The first supply catheter 12 is aflexible catheter having a diameter sufficiently small to allow itsdistal end to be inserted percutaneously into an accessible vein such asthe external jugular vein of a patient as shown in FIG. 7. The firstsupply catheter 12 is sufficiently long to allow the cooling element 14at the distal end of the first supply catheter 12 to be passed throughthe vascular system of the patient and placed in the superior vena cava62, inferior vena cava (not shown), or other such vein.

The method of inserting the catheter into the patient and routing thecooling element 14 into a selected vein is well known in the art.Percutaneous placement of the heat transfer element 14 into the jugularvein is accomplished directly, since the jugular vein is close to thesurface. The catheter would reside in the internal jugular and into thesuperior vena cava or even the right atrium.

Although the working fluid supply 10 is shown as an exemplary coolingdevice, other devices and working fluids may be used. For example, inorder to provide cooling, freon, perflourocarbon, water, or saline maybe used, as well as other such coolants.

The cooling element can absorb up to or more than 300 Watts of heat fromthe blood stream, resulting in absorption of as much as 100 Watts, 150Watts, 170 Watts or more from the brain.

Heating Blankets

FIG. 7 also shows a heating element 66, shown as a heating blanket.Heating blankets 66 generally are equipped with forced warm-air blowersthat blow heated air through vents in the blanket in a direction towardsthe patient. This type of heating occurs through the surface area of theskin of the patient, and is partially dependent on the surface areaextent of the patient. As shown in FIG. 7, the heating blanket 66 maycover most of the patient to warm and provide comfort to the patient.The heating blanket 66 need not cover the face and head of the patientin order that the patient may more easily breathe.

The heating blanket 66 serves several purposes. By warming the patient,vasoconstriction is avoided. The patient is also made more comfortable.For example, it is commonly agreed that for every one degree of corebody temperature reduction, the patient will continue to feelcomfortable if the same experiences a rise in surface area (skin)temperature of five degrees. Spasms due to total body hypothermia may beavoided. Temperature control of the patient may be more convenientlyperformed as the physician has another variable (the amount of heating)which may be adjusted.

As an alternative, the warming element may be any of the heating methodsproposed in U.S. patent application Ser. No. 09/292,532, filed on Apr.15, 1999, and entitled “Isolated Selective Organ Cooling Method andApparatus”, and incorporated by reference above.

The practice of the present invention is illustrated in the followingnon-limiting example.

Exemplary Procedure

-   1. The patient is initially assessed, resuscitated, and stabilized.-   2. The procedure may be carried out in an angiography suite or    surgical suite equipped with fluoroscopy.-   3. An ultrasound or angiogram of the superior vena cava and external    jugular can be used to determine the vessel diameter and the blood    flow; a catheter with an appropriately sized heat transfer element    can be selected.-   5. After assessment of the veins, the patient is sterilely prepped    and infiltrated with lidocaine at a region where the femoral artery    may be accessed.-   6. The external jugular is cannulated and a guide wire may be    inserted to the superior vena cava. Placement of the guide wire is    confirmed with fluoroscopy.-   7. An angiographic catheter can be fed over the wire and contrast    media injected into the vein to further to assess the anatomy if    desired.-   8. Alternatively, the external jugular is cannulated and a 10-12.5    french (f) introducer sheath is placed.-   9. A guide catheter is placed into the superior vena cava. If a    guide catheter is placed, it can be used to deliver contrast media    directly to further assess anatomy.-   10. The cooling catheter is placed into the superior vena cava via    the guiding catheter or over the guidewire.-   11. Placement is confirmed if desired with fluoroscopy.-   12. Alternatively, the cooling catheter shaft has sufficient    pushability and torqueability to be placed in the superior vena cava    without the aid of a guide wire or guide catheter.-   13. The cooling catheter is connected to a pump circuit also filled    with saline and free from air bubbles. The pump circuit has a heat    exchange section that is immersed into a water bath and tubing that    is connected to a peristaltic pump. The water bath is chilled to    approximately 0° C.-   14. Cooling is initiated by starting the pump mechanism. The saline    within the cooling catheter is circulated at 5 cc/sec. The saline    travels through the heat exchanger in the chilled water bath and is    cooled to approximately 1° C.-   15. The saline subsequently enters the cooling catheter where it is    delivered to the heat transfer element. The saline is warmed to    approximately 5-7° C. as it travels along the inner lumen of the    catheter shaft to the end of the heat transfer element.-   16. The saline then flows back through the heat transfer element in    contact with the inner metallic surface. The saline is further    warmed in the heat transfer element to 12-15° C., and in the    process, heat is absorbed from the blood, cooling the blood to    30° C. to 35° C. During this time, the patient may be warmed with an    external heat source such as a heating blanket.-   17. The chilled blood then goes on to chill the body. It is    estimated that less than an hour will be required to cool the brain    to 30° C. to 35° C.-   18. The warmed saline travels back the outer lumen of the catheter    shaft and is returned to the chilled water bath where the same is    cooled to 1° C.-   19. The pressure drops along the length of the circuit are estimated    to be between 1 and 10 atmospheres.-   20. The cooling can be adjusted by increasing or decreasing the flow    rate of the saline. Monitoring of the temperature drop of the saline    along the heat transfer element will allow the flow to be adjusted    to maintain the desired cooling effect.-   21. The catheter is left in place to provide cooling for, e.g., 6-48    hours.

Referring to FIG. 8, an alternative embodiment is shown in which theheat transfer element 14 is disposed in the superior vena cava 62 fromthe axillary vein rather than from the external jugular. It isenvisioned that the following veins may be appropriate to percutaneouslyinsert the heat transfer element: femoral, internal jugular, subclavian,and other veins of similar size and position. It is also envisioned thatthe following veins may be appropriate in which to dispose the heattransfer element during use: inferior vena cava, superior vena cava,femoral, internal jugular, and other veins of similar size and position.

FIG. 9 shows a cross-section of the heart in which the heat transferelement 14 is disposed in the superior vena cava 62. The heat transferelement 14 has rotating helical grooves 22 as well as counter-rotatinghelical grooves 24. Between the rotating and the counter-rotatinggrooves are bellows 27. It is believed that a design of this naturewould enhance the Nusselt number for the flow in the superior vena cavaby about 5 to 80.

Thermoregulatory Drugs

The above description discloses mechanical methods of rewarming apatient, or portions of a patient, to minimize the deleteriousconsequences of total body hypothermia. Another procedure which may beperformed, either contemporaneous with or in place of mechanicalwarming, is the administration of anti-vasoconstriction andanti-shivering drugs. Such drugs minimize the effect of vasoconstrictionwhich may otherwise hinder heat transfer and thus cooling of thepatient. In general, hypothermia tends to trigger aggressivethermoregulatory defenses in the human body. Such drugs also prohibitresponses such as shivering which may cause damage tocardiac-compromised patients by increasing their metabolic rate todangerous levels.

To limit the effectiveness of thermoregulatory defenses duringtherapeutic hypothermia, drugs that induce thermoregulatory tolerancemay be employed. A variety of these drugs have been discovered. Forexample, clonidine, meperidine, a combination of clonidine andmeperidine, propofol, magnesium, dexmedetomidine, and other such drugsmay be employed.

It is known that certain drugs inhibit thermoregulation roughly inproportion to their anesthetic properties. Thus, volatile anesthetics(isoflurane, desflurane, etc.), propofol, etc. are more effective atinhibiting thermoregulation than opioids which are in turn moreeffective than midazolam and the central alpha agonists. It is believedthat the combination drug of clonidine and meperidine synergisticallyreduces vasoconstriction and shivering thresholds, synergisticallyreduces the gain and maximum intensity of vasoconstriction andshivering, and produces sufficient inhibition of thermoregulatoryactivity to permit central catheter-based cooling to 32° C. withoutexcessive hypotension, autonomic nervous system activation, or sedationand respiratory compromise.

These drugs may be particularly important given the rapid onset ofthermoregulatory defenses. For example, vasoconstriction may set in attemperatures of only ½ degree below normal body temperature. Shiveringsets in only a fraction of a degree below vasoconstriction.

The temperature to which the blood is lowered may be such thatthermoregulatory responses are not triggered. For example,thermoregulatory responses may be triggered at a temperature of 1-1½degrees below normal temperature. Thus, if normal body temperature is37° C., thermoregulatory responses may set in at 35° C. Thermoregulatorydrugs may used to lower the temperature of the thermoregulatory triggerthreshold to 33° C. Use of the heating blankets described above mayallow even further cooling of the patient. For example, to lower thepatient's temperature from 33° C. to 31° C., a 2° C. temperaturedifference, a 2 times 5° C. or 10° C. rise is surface temperature may beemployed on the skin of the patient to allow the patient to not “feel”the extra 2° C. cooling.

A method which combines the thermoregulatory drug methodology and theheating blanket methodology is described with respect to FIG. 11. Thisfigure is purely exemplary. Patients' normal body temperatures vary, asdo their thermoregulatory thresholds.

As shown in FIG. 11, the patient may start with a normal bodytemperature of 37° C. and a typical thermoregulatory threshold of 35° C.(step 102). In other words, at 35° C., the patient would begin to shiverand vasoconstrict. A thermoregulatory drug may be delivered (step 104)to suppress the thermoregulatory response, changing the thresholdtemperature to, e.g., 35° C. This new value is shown in step 106. Theheat transfer element may then be placed in a high flow vein, such asthe superior or inferior vena cavae or both (step 108). Cooling mayoccur to lower the temperature of the blood (step 110). The cooling maybe in a fashion described in more detail above. The cooling results inthe patient undergoing hypothermia and achieving a hypothermictemperature of, e.g., 33° C. (step 112). More cooling may be performedat this stage, but as the thermoregulatory threshold has only beensuppressed to 33° C. (step 112), shivering and vasoconstriction woulddeleteriously result. This may complete the procedure. Alternatively, anadditional drug therapy may be delivered to further lower thethermoregulatory threshold.

An alternate way to lower the thermoregulatory threshold is to use aheating blanket. As noted above, a common rule-of-thumb is that apatient's comfort will stay constant, even if their body temperature islowered 1° C., so long as a heating blanket, 5° C. warmer than theirskin, is applied to a substantial portion of the surface area of thepatient (step 114). For a 2° C.-body temperature reduction, a 10° C.(warmer than the skin temperature) blanket would be applied. Of course,it is also known that blankets warmer than about 42° C. can damagepatient's skins, this then being an upper limit to the blankettemperature. The patient's body temperature may then continue to belowered by use of a heating blanket. For each 1° C. reduction in bodytemperature (step 116), the heating blanket temperature may be raised 5°C. (step 118). After each reduction in body temperature, the physicianmay decide whether or not to continue the cooling process (step 120).After cooling, other procedures may be performed if desired (step 122)and the patient may then be rewarmed (step 124).

It is important to note that the two alternate methods ofthermoregulatory response reduction may be performed independently. Inother words, either thermoregulatory drugs or heating blankets may beperformed without the use of the other. The flowchart given in FIG. 11may be used by omitting either step 104 or steps 114 and 118.

Vasoconstrictive Therapies

FIG. 10 showed the more rapid response of the high blood flow organs tohypothermia than that of the peripheral circulation. This response maybe maintained or enhanced by applying, as an alternative method ofperforming hypothermia, a cooling blanket rather than a heating blanket.The cooling blanket may serve to vasoconstrict the vessels in theperipheral circulation, further directing blood flow towards the heartand brain.

An alternate method of performing the same function is to provideseparate vasoconstrictive drugs which affect the posterior hypothalamusin such a way as to vasoconstrict the peripheral circulation whileallowing heart and brain circulation to proceed unimpeded. Such drugsare known and include alpha receptor type drugs. These drugs, as well asthe cooling blankets described above, may also enhance counter-currentexchange, again forcing cooling towards the heart and brain. Generally,any drug or cooling blanket that provides sufficient cooling to initiatea large scale cutaneous peripheral vasoconstrictive response would becapable of forcing the cooling blood flow towards the brain and heart(i.e., the “central” volumes). In this application, the term “peripheralcirculation” or “peripheral vasculature” refers to that portion of thevasculature serving the legs, arms, muscles, and skin.

Additional Therapies

Turning now from thermoregulatory drugs to additional therapies, themethod and device according to the embodiments of the invention may alsoplay a significant role in treating a variety of maladies involving celldamage.

Stroke

A patent application incorporated by reference above discloses devicesand methods for enhancing fibrinolysis of a clot by cooling bloodflowing in an artery. The present invention may also use blood coolingto substantially reduce platelet aggregation as there is a significantreduction in platelet activity at reduced temperatures. Such reductionmay take place by inhibiting enzyme function, although the actualmethodology is unclear. This reduction in platelet aggregation, as wellas the enhanced fibrinolysis noted above, may reduce or eliminatecurrent dependence on such drugs as tPA or Rheopro.

Myocardial Infarction

The above-described venous cooling may also provide a number of benefitsfor patients undergoing myocardial infarction.

Current therapies for treating myocardial infarction involve threeareas. Thrombolysis or stenting are used to establish reflow. The oxygensupply is increased by directly supplying the patient with oxygen and byvasodilation with nitrates. And the oxygen demand is lessened bydecreasing the heart rate and the blood pressure.

Devices and methods according to the present invention can work well incombination with these current therapies. For example, the device andmethod may lessen the heart's demand for oxygen by providing cooledblood to the heart. The cooled blood in turn cools the inner chambers ofthe heart, essentially from the inside. Hearts undergoing myocardialinfarction may beat very fast due to an agitated state of the victim.However, cooled blood may induce a state of bradycardia that reduces thedemand for oxygen by the heart per se.

To establish reflow and the oxygen supply, the enhanced fibrinolysis,discussed above, may also dissolve the clot, allowing more blood flowand more oxygen delivered to the heart. As mentioned above, plateletaggregation may be reduced. Additionally, conduction through thesubendocardium, cooling the heart, may reduce the overall metabolicactivity of the heart as well as protect the subendocardium from celldamage.

It is additionally noted that reflow is often accompanied by reperfusioninjury which can further damage cells. Neutrophil activation occurs aspart of reperfusion injury. Hypothermia can limit such activation andthus can limit reperfusion injury.

Thus, numerous therapies may be delivered by one device. Therefore,e.g., currently-employed “beta-blocker” drugs used to reduce heart ratein patients undergoing infarcts may not need to be employed in patientsundergoing these hypothermic therapies.

Re-Stenosis

Another application of the device and method may be in the treatment ofstenotic arteries. Stenotic arteries are vessels that have narrowed dueto a build-up of tissue and/or plaque atheroma. Stenotic vessels aretreated by angioplasty or stenting, which opens the artery. Duringtreatment the vessel wall may be injured. Such injuries often (20-50%)cause an inflammatory reaction that eventually causes the vessel toundergo re-stenosis after a period of time, which may range from 6-12months or even several years later.

Hypothermia is known to mitigate inflammatory responses. For example,one of the initial steps in the process of re-stenosis is the migrationof macrophages or white blood cells to the injured area. Hypothermia canlimit this migration. Hypothermia can also inhibit reactions andprocesses initiated by molecules acting in an autocrine or paracrinefashion. Hypothermia may also limit the release of several growthfactors (at the site of injury) such as PDGF and EGF that act in thesefashions.

While the invention herein disclosed is capable of obtaining the objectshereinbefore stated, it is to be understood that this disclosure ismerely illustrative of the presently preferred embodiments of theinvention and that no limitations are intended other than as describedin the appended claims.

1. A method for substantially reducing stenoses recurring followingangioplasty, comprising: providing a catheter having a cooling elementattached to a distal end thereof, the cooling element employingmixing-inducing surface features; performing an angioplasty or stentingoperation on a blood vessel of the patient; inserting the catheterthrough the vascular system of the patient to place the cooling elementin the blood vessel; circulating fluid through the cooling element; andtransferring heat from the blood in the vessel to the cooling element,whereby blood and the blood vessel wall are cooled.
 2. The method ofclaim 1, wherein said mixing-inducing surface features create aturbulence intensity of at least about 0.05.
 3. The method of claim 1,wherein said mixing-inducing surface feature is a helix.
 4. The methodof claim 1, wherein said mixing-inducing surface features include asubstantially cylindrical body having protrusions thereon.
 5. The methodof claim 1, wherein said mixing-inducing surface features create amixing characterized by a Nusselt number of at least about
 5. 6. Themethod of claim 1, wherein said mixing-inducing surface feature includesat least one helical invagination.
 7. The method of claim 1, whereinsaid mixing-inducing surface features include at least one protrusion.